Light sensitive apparatus



June 18, 1935. w. F. TEDHAM LIGHT SENSITIVE APPARATUS Filed Oct. 27, 1933 INVENTOR WILL/AM F. TED/664M v BY K g 7 Mv- L,

ATTORNEY Patented June 18, 1935 2,005,059 I'JGHT siiNsiTIvE APPARATUS Wil liam Fiaiic'is Tedham, liondon, England, assigner to Electric and Musical-Industries Lini- '-'ited, Hayes, 'Englanmfla company 'of Great Britain Applicatmnoctoberev, 1933, seriarivo. 695,381

I'n'Gieat' Britain November '11, 1932 5 Claims. fi' rnepresent invention relates to light sensitive apparatus andmore particularly to photo=electric de 1:

In order to increase themag'nitude of the photo.-

current developed when light of agiven intensity electric devices of type, the ,pri-

mary and secondaryicathodes have 'beenconstiltuted hitherto by plane, rectangular electrodes disposedparallel or inclined at an angle to one "snot :Lwith alplarieanode or. meshmaterial dispose-t1 symmetrically betweenth'em.

When oxidizing the surfaces of parallel plate electrodes, difficulty has been experienced in obtaining a uniform discharge in oxygen, at the usual pressures of 0.5 to 1 mm. of mercury, as the edge effects are serious. This lack'of uniformity is increased when the plates are inclined to one another and it has been found impossible to produce cathodes of high sensitivity when the surfaces thereof have been unevenly oxidized.

A further difiiculty is that the process of cathode formation following the introduction of the light sensitive material such as caesium, cannot readily be observed when the electrodes are disposed in the above described known manner. Hence the amplification produced by secondary emission in this known type of photo-electric device is virtually nullified by the low efficiency of the cathode which can be employed.

According to the present invention, there is provided a photo-electric device comprising a primary cathode adapted to emit electrons when subjected to illumination, a secondary cathode adapted to emit secondary electrons when electrons from the primary cathode impinge thereon, and an anode disposed between said primary and secondary cathodes, wherein said primary or secondary cathode or both are of curved or dished shape, the concave side or sides being directed towards the anode.

Preferably the anode is in the form of a straight Wire or cylinder and thecathodes are arranged concentrically or 'co-axially with-respect to the anode and upon opposite sides thereof.

A preferred embodiment of the invention will now be described by way of example, reference being made to the accompanying drawing, in which device illustrated in Fig. 1.

' fFig. r15 anillustr'a'tion of a photo-electric as vice constructed in accordance with invention,'and

Fig. 2 is a plan view of the present Like'parts inthe'two figures erence numbers. A I

Referringnow to the two figures, aphoto-elecbear'the samer'ef 'tric device comprises 'an-evacuated glass envelope 1 having disposed'within, a primary cathodef2',}a

"the electrodesof the secondary cathode 3 and an anode 'fl. EacheIec- 'tro'de is'mountedinusualmanner, upon an independent support which is sealed through a pinch 5 in a re-entrant stem of the envelopefL,

The primary cathode "2 comprises ahollOW semi-cylinder, preferably of silver, the concave side of :whichis directed-towards" the anode lfthe latter"beingconstituted by 'arigid filament 16'- the side of the anode 4 remote from the primary cathode 2 and is arranged co-axially therewith. The diameter of the secondary cathode 3 is considerably less than that of the primary cathode 2. It may conveniently be about one-quarter that of the primary cathode 2.

In one tube constructed in accordance with the invention, the anode and secondary cathode are maintained at potentials of about 400 and 300. volts respectively relative to the primary cathode;

If desired, the primary and secondary cathodes may be in the form of concentric hemispherical shells and the anode may then be in the form of a small ball held at the center of the shells. A central horizontal section of such an arrangement would then be as shown in Fig. 2.

Whichever arrangement of electrodes is adopted the concave surfaces of the two cathodes are sensitized by oxidation and subsequent deposition of caesium. or other suitable material. Oxidation may beeffected in known manner by introducing oxygen into the envelope l of the device to a pressure of 0.5 to 1 mm. of mercury and passing a high voltage, highfrequency or direct current discharge between the anode 4 and cathodes 2 and 3. If either or both of the cathodes is part-spherical in shape, it has been found preferable to employ a high frequency discharge. With part-cylindricalcathodes, a direct current discharge has been found to produce the better result. A layer of caesium is then deposited upon the oxidized surfaces of thecathode which light may-pass, in the direction indicated by the arrows, to the primary cathode 2.

In operation, when light falls upon the primary cathode 2, electrons are emitted and, under the;

influence of the electrostatic field, maintained between the electrodes, some of these electrons pass to and are collected by theanode 4 while others travel on to the secondary cathode 3 and.

there cause an increased number of secondary electrons to be emitted. These secondary electrons travel to the anode 4 and augment the total number of electrons impinging thereon.

With the electrodes arranged as described above, uniform oxidation of the cathodes'is possible and, in addition, formation of the light sensitive surface may be readily observed and the incid'ent light is not impeded to any great extent by the secondary cathode.

The cathodes'need not be of accurately hemispherical or. semi-cylindrical form but may be of any dished shape similar to those described above.

Having now particularly I described and ascertained the nature of my said invention, and in what manner the same is to be performed, I declare that what I claim is:

1. A photo-electric device comprising a primary cathode adapted to emit electrons when subjected to illumination, a secondary cathode .adapted to emit secondary electrons when electrons from the primary cathode impinge thereon, and ananode disposed between said primary and secondary cathodes, both of said cathodesbeing curved and mounted with their concave sides directed toward each other and towards the anode.

2. A photo-electric device as claimed in claim 1, wherein the anode is in the form of a thin rod, and the primary and secondary cathodes are in the form of hollow semicylinders. p

3. A light. sensitive device comprising a primary cathode adapted to emit electrons when subjected to illumination, a secondary cathode adapted to emit secondary electrons when electrons from the primary cathode impinge thereelectrons whensubjected to illumination, a semicylindrical secondary cathode having its concave side facingthe concave side of the primary cathode .adapted to emit secondary electrons. when electrons from the primary cathod impinge thereon, and an anode disposed between said primary and secondary cathodes, said cathodes being mounted coaxially with respect to oneanother. a 1 5. A light sensitive device comprising a semicylind'ric'al primary cathode adapted to emit electrons when subjected to i1lumination,.a semic'ylindrical secondary cathode adapted to emit secondary electrons when electrons from the primary cathode impinge thereon, and an'anode disposed between said, primary and secondary cathodes," the diameter of the cylinder. in which the surface of said primary cathode lies being greater than the diameter 01' the 'cylinder'in which the surface of said secondary cathode lies.

' WILLIAMFRANCIS TEDI -IAM. 

